1. Field of the Invention
The present invention relates to equipment and a process for wrapping a pallet load with a stretchable wrapping material in such a manner so as to achieve the tightest wrap possible without damaging the load being wrapped.
2. Description of the Prior Art
United Kingdom Pat. No. 2,059,906 discloses a stretch wrapping machine and process. An accumulator having a dancer roller which contacts the moving film, compensates for variations in speed at which the film is drawn into the object being wrapped. The dancer roller is biased against the moving film by a constant force producing element.
Several different types of stretch wrapping machines are illustrated in U.S. Pat. Nos. 4,050,221; 4,077,179; and 4,079,565, all to Lancaster et al. Other stretch wrapping machines are shown in U.S. Patent Applications Ser. No. 72,471, filed Sept. 4, 1979 to Humphrey, and Ser. No. 235,946, filed Feb. 19, 1981 to Humphrey et al, both of which are assigned to the same assignee as the present application. The subject matter of both of these applications is hereby incorporated by reference. Stretch wrapping a pallet load may be compared to stretching a rubber band around a group of objects. Presuming that there is a uniformly-shaped pallet load that has been stretch wrapped with a stretchable wrapping film tensioned to a 10 pound pull and that there are three wraps, i.e. the load has been wrapped three times around the load, then the force holding the load together are 30 pounds in both directions at each corner. At the center points of each side of the load there is no direct inward holding force although the products in the center of the load are clamped together by the adjacent outer products. There is also a diagonal force which is the resultant of the two directional forces on each corner of the load.
It must be recognized that all wrapping films relax with the result that the rubber band effect or holding power is diminished with the passing of time. The amount of tension required for a particular load must be sufficient to contain the integrity of the load which may settle, change shape or shift in transit or during storage. A shifting load alternately stretches and relaxes the film; each time the film is stretched further, the recovery tension is reduced. Enough initial tension must be applied to the load to compensate for these subsequent.
If the extent to which the wrapping material is stretched is too great, then this will diminish its holding power. Most films produce their greatest holding force or tension when stretched during the wrapping operation between approximately 20% to 35%. If a film is stretched beyond its elastic limit, which is the point where permanent deformation occurs, the film thins out in gauge and its ability to recover and hold the load will decrease, perhaps drastically and perhaps even destroyed. For example, a wrapping film with a maximum holding power of 30 pounds when stretched to 30% stretch may have only 15 to 17 pounds holding power if stretched over 100%. If the resulting weaker film is adequate to hold the load securely then economies will result by using less film per pallet load. Otherwise, additional wraps will be required to obtain the same holding force as can be obtained by fewer of the same film stretched only 20% to 35%.
As indicated above, all stretch films relax to some extent, in varying degrees, after the load is wrapped. In addition, the wrapping films which have been stretched or prestretched more than 100% will tend to relax more than films which are stretched 20% to 35% during the wrapping process. The objective in wrapping a load with a stretchable wrapping material is to obtain the tightest wrap possible without damaging or causing collapse of the load.
The concept of prestretching the stretchable wrapping material before wrapping a load has recently emerged and has been incorporated in several commercial machines. Employing a film prestretching device makes the film longer and thinner while simultaneously increasing the yield and decreasing the load holding power or strength of the film. Since various loads, however, may need to be wrapped with film having different holding powers, prestretching enables a single type of film to be used in wrapping a plurality of different types of loads with the film then being prestretched to the extent necessary.
Whether or not a prestretching device is used, however, the film must still be stretched by applying tension on the film as it is applied to the load. This tension, which results from the load pulling the film against some restraining device during the wrapping operation, is normally necessary since it causes the stretching that provides the holding power to hold the load together. Prestretching is a separate and isolated function from stretch wrapping of the load. Whether film prestretching is done on the wrapping machine or in the film manufacturers plant, the film normally must still be further stretched during the wrapping operation.
The currently available commercial prestretching devices normally consist of two rubber-covered rollers which are rotated at different speeds. The speed differential is created by appropriate gears, belt drives, separate D.C. variable speed motors, separate brakes or other similar types of mechanisms. While some of the prestretching devices are powered by the load pulling the film, most of the devices are motor powered.
In the operation of the pre-stretching devices, the film passes over both rollers with the second roller rotating at a faster speed than the first roller thereby providing a stretching action on the film. If the second roller rotates twice as fast as the first, assuming there is no slippage and a minimum "necking down" of the film, the film will be stretched approximately 100%. Various speed ratios of the rollers will produce proportional percentage stretch in the film. For example, a relatively heavy gauge film (#90) may theoretically be doubled in yield (length) to wrap light loads with light tension. The original holding power of such film was 30 pounds but with 100% pre-stretching the gauge changes to #45 and the holding power is approximately 16 pounds, which is adequate for light loads.
Loads consisting of cartons which are easily crushed may be stretch wrapped with light gauge film under very low tension. Light gauge films are on the market in the range of 60 to 70 gauge but these films cost more per pound. The pre-stretching devices permit the use of lower cost heavy gauge films which may be pre-stretched to make light gauge films which are then wrapped around the load under light tension. For example, a 5000 foot roll of 100 gauge film, if pre-stretched to 100% would yield 10,000 feet of 50 gauge film and wrap twice as many pallet loads which may be adequately wrapped under light tension.
There are several different types of pre-stretching systems. A first type is a non-powered system which relies upon the load pulling the film both off the roll of film and through the pre-stretching device. This system eliminates the requirement for friction brakes but considerable inertia is added to the operation of the system both due to the necessity of pulling the material off of the film roll and due to the need to rotate the rubber rollers which are geared together. In addition, the gears between the rollers must be changed in order to change the percentage of pre-stretching.
A second type of system is a powered system that employs a variable speed motor drive for each of the two rubber rollers plus a friction brake on the film roll. The two motors must attempt to maintain a speed differential under varying film demands which often leads to severe inaccuracies in the extent of pre-stretching. In addition, the friction brake adds inherent erratic behavior toward trying to maintain a constant pre-stretching.
A second type of powered system uses one power driven roll with a friction brake then being coupled to the other roller. The pre-stretching is adjustable by varying the setting of the electromagnetic brake torque of the friction brake.
In all of the above systems, however, there is still the need for additional stretching of the film to take place between the pre-stretching device and the load when actually wrapping the load. This additional stretching during the wrapping operation quite often varies in magnitude due to the uneven shape of the load, which is normally not round. The final result is that each load is still wrapped under a different stretch tension as well be further explained below.
While there are several benefits of pre-stretching, such as outlined above, there are also definite limitations to such procedures. In most stretch wrapping applications, the integrity of the unitized load depends upon the cling of one layer of film to another layer of film. In addition to keeping the tail end of the wrap from unwrapping, film cling is extremely important in spiral wrapping where the "lamination" or overlaying of one layer upon another produces considerable strength in the wrapped load. Many of the stretchable films currently on the market tend to dramatically lose their clinging ability when stretched beyond 100%. Often loads that are stretch wrapped with 120% pre-stretched film have been observed to become unwrapped in 48 hours. Other films will become too brittle and shatter like glass when excessively pre-stretched while yet other films will take a set and lose their load holding power. All films tested have demonstrated a considerable loss in strength in the transverse direction when stretched over 100%.
This inherent limitation in the extent to which it is beneficial to pre-stretch a film prior to wrapping becomes even more critical upon realizing that further variable stretching occurs during the wrapping operation itself. The rotating turntable support member rotates a load which is usually not round. This results in variable demands on the film dispensing mechanism. This becomes a particular problem where the dispensing of stretchable material does not occur at an identical rate with the demand for such material by the rotating load. This is particularly a problem where a pre-stretching device is inserted between the film roll and the load since the roll of film does not roll freely in response to the taking up of film by the load. Inherently this variable demand for film that is not synchronized with the dispensing mechanism produces an erratic tensioning of the film.
As can be seen from FIG. 8 of the drawings, which illustrates a prior art system, with a rectangular load the demand for film will vary greatly as the load is rotated. Assuming that the pallet load size is 40 inches by 60 inches when the film is wrapping the long side of the load, the effective wrapping diameter is 40 inches. At 10 rpm turntable speed, the film is drawn from the roll at 105 fpm. As the corner of the load approaches, the effective wrapping diameter becomes the diagonal dimension of 72 inches. At 10 rpm turntable speed, the film must now accelerate to 188 fpm in less than 1 second, i.e. an 80% increase in speed. Subsequent rotation of the load produces a similar decrease in speed and a series of sudden accelerations and decelerations as the load continues to be wrapped. The film speed curve abruptly changes during the wrapping operation. The inertia and momentum of the film roll constantly opposes the desired results by tightening and slackening film tension during the wrapping cycle which further enhances the degree of variance in tension of the wrapping film. In such a wrapping operation, the film tension at the corners of the load can double with the film brake turned off due entirely to the inertia of the film roll. The peak in film tension occurs just as the sharp corner of the load is presented to the film. Consequently, the tension on the film must be set well below (approximately half) the theoretical limits of the film to prevent breaking of the film.
This problem is further aggravated by the braking systems in common use today. Most of the braking systems employ some variation of a friction brake, usually electro-magnetically controlled. An electromagnetic brake has approximately 300 to 400% more torque at rest (static) than when rotating. This means in connection with the operation of the stretch wrapper that when the film tension drops off suddenly during load rotation, the film roll stops turning momentarily and the brake becomes tightly locked. As the corner of the load swings outwardly (see FIG. 8) the film tension suddenly increases and the friction brake must be jerked into rotation to reduce its braking torque to preset value. This violent action occurs just as a relatively sharp corner of the load comes around and it is one of the major causes of film breakage. This problem becomes even more pronounced with higher speeds of rotation.